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Energy Efficiency -
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Solutions  Until recently, clear glass was the primary glazing material used in windows. Although glass is durable and allows a high percentage of sunlight to enter buildings, it has very little resistance to heat flow. During the past two decades, though, glazing technology has changed greatly. Research and development into types of glazing have created a new generation of materials that offer improved window efficiency and performance for consumers. |
| While this new generation of glazing materials quickly gains acceptance in the marketplace, the research and development of even more efficient technologies continues. |
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Current Options that Increase a Window's Energy Efficiency |
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Manufacturers usually represent the energy efficiency of windows in terms of their U-values (conductance of heat) or their R-values (resistance to heat flow). If a window's R-value is high, it will lose less heat than one with a lower R-value. Conversely, if a window's U-value is low, it will lose less heat than one with a higher U-value. In other words, U-values are the reciprocals of R-values (U-value = 1/R-value). |
 Low-e Glazings Low-e glazings have special coatings that reduce heat transfer through windows. The coatings are thin, almost invisible metal oxide or semiconductor films that are placed directly on one or more surfaces of glass or on plastic films between two or more panes. The coatings typically face air spaces within windows and reduce heat flow between the panes of glass. |
When applied inside a double-pane window, the low-e coating is placed on the outer surface of the inner pane of glass to reflect heat back into the living space during the heating season. This same coating will slightly reduce heat gain during the cooling season. Low-e films are applied in either soft or hard coats. Soft-coat low-e films degrade when exposed to air and moisture, are easily damaged, and have a limited shelf life, so they are carefully applied by manufacturers in insulated multiple-pane windows. Hard low-e coatings, on the other hand, are more durable and can be used in add-on (retrofit) applications. But the energy performance of hard-coat low-e films is slightly poorer than that of soft-coat films. Windows manufactured with low-e films typically cost about 10% to 15% more than regular windows, but they reduce energy loss by as much as 30% to 50%.
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Tomorrow's Options for More Efficient Windows |
| "Superwindows" now coming on the market can attain high thermal resistance by combining multiple low-e coatings; low-conductance gas fills; barriers between panes, which reduce convective circulation of the gas fill; and insulating frames and edge spacers. Also, optical properties such as solar transmittance can be customized for specific climate zones. The heat from even a small amount of diffuse winter sunlight will convert these super-windows into net suppliers of energy. This first generation of superwindows now available have a center-of-glass R-value of 8 or 9, but have an overall window R-value of only about 4 or 5 because of edge and frame losses. Also under development are chromogenic (optical switching) glazings that will adapt to the frequent changes in the lighting and heating or cooling requirements of buildings. These "smart windows" will be separated into either passive or active glazing categories. Passive glazings will be capable of varying their light transmission characteristics according to changes in sunlight (photochromic) and their heat transmittance characteristics according to ambient temperature swings (thermochromic). Active (electrochromic) windows will use a small electric current to alter their transmission properties. Both types should be on the market within 2 to 5 years. |
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Conclusion |
| No one type of glazing is suitable for every application. Many materials are available that serve different purposes. Moreover, consumers may discover that they need two types of glazing for a home because of the directions that the windows face and the local climate. To make wise purchases, consumers should first examine their heating and cooling needs and prioritize desired features such as daylighting, solar heating, shading, ventilation, and aesthetic value. |
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Source List |
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The following organizations and publications provide more information on advances in glazing technology. EREC provides free general and technical information to the public on the many topics and technologies pertaining to energy efficiency and renewable energy. Efficient Windows Collaborative Provides unbiased information on the benefits of energy-efficient windows, descriptions of how they work, and recommendations for their selection and use. National Fenestration Rating Council (NFRC) Developed the Procedure for Determining Fenestration Product Thermal Properties (NFRC 100-91). These procedures are now being used in NFRC's window certification and efficiency labeling programs, which have already been adopted by three states. Provides technical support to government and industry efforts to help architects, engineers, and other commercial building specifiers choose energy-efficient and cost-effective residential windows. Window & Door Manufacturers Association A trade association representing U.S. and Canadian manufacturers and suppliers of windows and doors for the domestic and export markets. |
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Reading List
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